Permanent magnet synchronous motors are used where absolute speed is required in various types of instruments, computers, music reproducing, and hospital equipment.
The typical permanent magnet synchronous motor wiring practice is to place a capacitor in series with the phase shifted motor oil. The selected value of that capacitor determines the phase shift angle, the current flow and voltage to that coil, and the performance behavior of the motor as well as directionability. In some instances, an additional external resistor is put in series with the capacitor to refine motor characteristics by limiting or adjusting current flow through the phase shifted coil.
Even under optimum selections of the capacitor and resistor, roughness affecting angular acceleration and deceleration of the rotor shaft creates gear train noise problems in many applications requiring the use of such motors.